Over the last two decades metallic glasses (MGs) have received increasing attention because of their unique characteristics, such as high strength, high specific strength, large elastic strain limit, excellent wear and corrosion resistance, along with other remarkable engineering properties. (For further discussion see, e.g., A. L. Greer, Science 1995, 267, 1947; W. L. Johnson, MRS Bulletin 1999, 24, 42; A. Inoue, Acta Materialia 2000, 48, 279; D. H. Xu, G. Duan, and W. L. Johnson, Physical Review Letters 2004, 92, 245504; V. Ponnambalam, et al., Journal of Materials Research 2004, 19, 1320; and Z. P. Lu, C. T. Liu, J. R. Thompson, W. D. Porter, Physical Review Letters 2004, 92, 245503, the disclosures of which are incorporated herein by reference.) Because of the promise shown by these materials, researchers have designed a multitude of multi-component systems that form amorphous glassy alloys, among which Zr— (U.S. Pat. No. 5,288,344, referred to as Vit1 series of alloys, the disclosure of which is incorporate herein by reference) bulk metallic glasses (BMGs) have been utilized commercially to produce a variety of items, including, for example, sporting goods, electronic casings, and medical devices.
Most practical applications of MGs demand near-net-shaping process in manufacturing. However, conventional die casting, the common technique for net-shape processing of metals, requires fast cooling to bypass the crystallization of most MGs during solidification. This fast cooling requirement limits the ability to make pieces of large cross-section (i.e., limited by critical casting thickness), limits the ability to make parts with high aspect ratios (i.e., with large thin walls), and limits the ability to make high quality casts or to manufacture structures with complex geometries. Nevertheless, the properties of these MGs, including their high glass forming ability, good processability, large supercooled liquid region (SCLR), and a viscosity that varies continuously and predictably in the supercooled liquid region continues to hold out the promise that they could be processed thermoplastically if suitable candidate materials can be identified.
The unique advantages of injection molding, blow molding, micro-replication, and other thermoplastic technologies are largely responsible for the widespread uses of plastics such as polyethylene, polyurethane, PVC, etc., in a broad range of engineering applications. Powder Injection Molding (PIM) of metals represents an effort to apply similar processing to metals, but requires blending of the powder with a plastic binder to achieve net shape forming and subsequent sintering of the powder. Given suitable materials, thermoplastic forming (TPF) would be the method of choice for manufacturing of metallic glass components because TPF decouples the forming and cooling steps by processing glassy material at temperatures above the glass transition temperature (Tg) and below the crystallization temperature (Tx) followed by cooling to ambient temperature. (See, e.g., J. Schroers, JOM 2005, 57, 35; and J. Schroers, N. Paton, Advanced Materials & Processes 2006, 164, 61, the disclosures of which are incorporated herein by reference.)
Thermoplastic forming (TPF) of MGs is a net-shaping processing method taking place in the supercooled liquid region of such materials, which is the temperature region in which the amorphous material first relaxes into a viscous metastable liquid before crystallization. Operating in this supercooled liquid region, TPF decouples the fast cooling and forming of MG parts and allows for the replication of small features and thin sections of metals with high aspect ratios. TPF has several advantages over conventional die casting, including smaller solidification shrinkage, less porosity of the final product, more flexibility on possible product sizes, a robust process that does not sacrifice the mechanical properties of the material, and no cooling rate constraints on the thickness of parts that can be rendered amorphous (critical casting thickness).
From a processing point of view, MG alloys with an extremely large supercooled liquid region (excellent thermal stability against crystallization), which can provide lower processing viscosities and exhibit smaller flow stress, would be desirable for use in conjunction with a TPF process. In addition, excellent glass forming ability and low glass transition temperature (Tg) are also preferred to thermoplastically process MGs. Unfortunately, among the published metallic glasses, only the expensive Pt-, and Pd-based glasses have shown good thermoplastic formability. (See, e.g., J. Schroers, W. L. Johnson, Applied Physics Letters 2004, 84, 3666; G. J. Fan, et al., Applied Physics Letters 2004, 84, 487; and J. P. Chu, et al., Applied Physics Letters 2007, 90, 034101, the disclosures of which are incorporated herein by reference.) Zr-based metallic glasses, especially the Vitreloy series, are much less expensive than Pt- and Pd-based alloys, have exceptional glass forming ability, but they are usually strong liquids (the drop of viscosity with temperature is not steep) and low processing viscosities are unattainable in the supercooled liquid region (SCLR) between Tg and Tx. (See, e.g., A. Masuhr, et al., Physical Review Letters 1999, 82, 2290; R. Busch, W. L. Johnson, Applied Physics Letters 1998, 72, 2695; F. Spaepen, Acta Metallurgica 1977, 25, 407; and J. Lu, G. Ravichandran, W. L. Johnson, Acta Materialia 2003, 51, 3429, the disclosures of which are incorporated herein by reference.) One exception to this general rule is Vit1b (Zr44Ti11Cu10Ni10Be25); however, even this allow only provides accessible viscosities of ˜10^Pa-s, substantially higher than the viscosities needed to access most thermoplastic forming techniques. (See, Schroers, J., et al. Scripta Materialia, 2007, 57, 341-344.1
Accordingly, a need exists for a new family of inexpensive MGs that can be incorporated into a thermoplastic processing application.